The invention relates to an optical sensor element for detecting organic compounds, particularly hydrocarbon, which can be utilized, for example, in environmental and health protection.
Hydrocarbon may pollute the environment and may be harmful to human health. Hence, the measurement of hydrocarbon is an important task. Conventional methods for detecting hydrocarbon are chromatography, IR-absorption methods, acousto-optic measuring methods, heat conductivity measurements, heat effect measurements of the catalytic decomposition of the hydrocarbons, or electrolytic conductivity measurements.
The first three measurement methods use expensive equipment and only permit application in laboratories. The other methods are suited for embodying measuring feelers which can be installed at different places in the environment or industrial processes. Their lifetime is short when subject to corrosive ambiance. Since their function relies on electrical principles they are, when used in an explosive environment, only applicable with explosion-protective measures. The measuring and amplifier electronic, respectively, has to be installed in the direct vicinity of the measuring feeler. In contrast thereto, fiber-optical sensors conventionally do not require any additional explosion-protective measures and they permit greater distances, up to the kilometer range, between the measuring location and the signal evaluation (remote sensing) due to the extremely interference-safe and low attenuated measuring signal transmittance via light cable. Thus, in contrast to electronic sensors, applications become feasible even under adverse environmental conditions and at measuring places difficult to access. Recently, solutions of fiber-optical sensors have become known for detecting hydrocarbon.
So it has been proposed, for example, to make light cables out of porous glass and, in a next step, to secure a chemical species, which is sensitive towards the substances to be detected to the glass surface [M. Tabacco et al: "Chemical Sensors for Environmental monitoring"; SPIE Vol. 1587 Chemical, Biochemical and Environmental Fiber Sensors III (1991) 271]. However, a such prepared porous fiber is scarcely suited as a sensor in practical use since it is very fragile. Furthermore, with this sensor principle, the intensity measurements on light conducting fibers are an ill suited means to ensure a stable and quantitatively reliable detection of the concentration of the substances to be detected since. Any variation of the light intensity of the light source or of the fiber feed to the sensor distort the measuring signal.
Other examples use a light conducting fiber which is composed of a quartz glass core and an optical coat made of silicon [J. P. Conzen, et al: Characterization of a Fiber-Optic Evanescent Wave Absorbance Sensor for Nonpolar Organic Compounds"; Applied Spectroscopy Vol. 47, 6 (1993) 753 or C. Ronot, et al: "Detection of chemical vapours with a specifically coated optical fibre sensor"; Sensors and actuators B, 11 (1993) 375-381]. The silicon coat protects the fiber core against water and other polar substances. Organic compounds, however, such as hydrocarbon compounds, are able to diffuse into the silicon coat. Refractive index variations, swelling and optical absorption variations may result. These variations affect the transmission of the light conducting fiber, since a definite portion of the light conducted in the light conducting fiber also enters into the optical coat as a so-called evanescent wave where it is subject to the variations. Sensors based upon these effects mostly require light conducting fibers of several meters to ensure a sufficient sensor sensitivity and, hence, some space, since the evanescent light portion only makes a small part of the entire light conducted. In this case, it is also true that the principle of transmission measurement on light conducting fiber sensors is disadvantageous with respect to the stability and reproducibility of the sensors. Such disadvantages may deteriorate the advantages otherwise inherent in optical fiber sensors, such as explosion safety, immunity to electromagnetic interference fields, optimal electric potential splitting or bridging of great distances between measuring place and the location of the electronic signal evaluation and, thus, may prevent practical use.
It is further known that a safe and non-distorted transmission of optical sensor signals is feasible with light conducting fibers, even over wide distances, when the measuring information is transmitted as a spectrally encoded optical signal wherein the variations of the measuring information are rendered measurable as spectral displacements of maxima or minima of the light intensity. An example of such a principle is a fiber-optical moisture sensor [EP 0 536 656 A1] and associated method for signal evaluation [EP 0 538 664 A2]. The proper moisture-sensitive element of the fiber-optical moisture sensor is an optical narrow-band filter constituted of a stack of sandwiched optical layers selected from inorganic dielectric material with alternating high and low optical refractive index. The optical layer thickness always is a multiple quarter and a multiple half, respectively, of an adjustable mean working wavelength of the measuring light. It has been known that such layers, when manufactured by vacuum deposition, are porous and, in the presence of water vapor in the ambiance, can take up water [H. Koch: "Optische Untersuchungen zur Wasserdampfsorption in Aufdampfschichten" phys. stat. sol. 12 (1965) 533-543]. The optical refractive indexes of the individual layers vary with the absorption of water and the filter spectrum is displaced towards longer wavelengths.
The spectral displacements of the filter spectra are very precisely measurable even over wide light conducting fiber paths and fiber optical moisture sensors provided with such sandwich stacks permit a very reliable evaluation.
However, such layers according to the state of the art are only sensitive with respect to water vapor, apart from small undesired transverse sensitivities of other vapors such as, for example, alcoholic vapor or ammonia.
It is an object of the invention to provide an optical sensor element for detecting organic compounds, particularly hydrocarbon, that permits the selective detection of the latter by use of units which produce spectral displacements.